The invention relates to laser control systems, and more particularly to a technique for establishing a common frequency and phase for a two-dimensional array of lasers which are characterized by a uniform nominal frequency but individual operating frequencies which vary from nominal.
High power phased-array injection lasers have recently become a target of investigation for researchers in the area of optoelectronics. One purpose of these investigations is to obtain a coherent high power output beam for various applications. If a coherent addition of the individual laser outputs into a single high power beam is desired, the frequencies and relative phases of the individual beams must be locked together. Unfortunately, no method has previously been found for controlling the relative phases of phased-array lasers. For example, with gallium arsenide (GaAs) lasers having a nominal frequency of approximately 10.sup.14 Hz, the typical frequency variation encountered among lasers formed from the same processing facility is on the order of several gigahertz, while GaAs lasers manufactured by different companies using different growth techniques can vary from the nominal frequency by hundreds of gigahertz. Often each laser itself selects phases so as to minimize its threshold current, and in many cases these phases are the undesirable ones. See, e.g., Streifer, et. al., "Phased Array Diode Lasers", Laser Focus, Page 100 (June 1984).
Semiconductor lasers such as GaAs lasers are rapidly assuming a prominent role among all lasers. Because of their small size and inherent design flexibility, semiconductor lasers are useful for wide areas of applications. The problem of phase locking the outputs from a multitude of semiconductor lasers is thus becoming increasingly important.
It has previously been found that, if a number of lasers are placed close enough together, an "evanescent" wave will be formed outside the confines of the individual beams, resulting in an energy coupling between adjacent lasers that causes the various lasers to lock together in frequency and phase, if they are close enough to each other. To date it has been possible to produce evanescent coupling only for linear arrays; some mechanism is still necessary to lock together the frequency and phase of the various lasers in a two-dimensional array.